1. Field of the Invention
The present invention relates to current sensors. In a particularly preferred embodiment, the present invention relates to a current transformer with a core that is actively reset to prevent continuous saturation. The present invention also relates to methods of sensing current.
2. Description of Related Art
Current transformers are widely used for measuring current. A current transformer comprises primary and secondary windings that are wound about a transformer core. A primary current I.sub.1, which is the current that the current transformer is used to measure, flows through the primary winding. The primary current I.sub.1 induces a magnetic flux which flows through the transformer core and which induces a secondary current I.sub.2 in the secondary winding. For a linear (unsaturated) current transformer, the primary current I.sub.1 multiplied by the number of turns N.sub.1 on the primary winding is equal to the secondary current I.sub.2 multiplied by the number of turns N.sub.2 on the secondary winding: EQU I.sub.1 N.sub.1 =I.sub.2 N.sub.2 (1)
Therefore, since the parameters N.sub.1 and N.sub.2 are known, the primary current I.sub.1 may be measured indirectly by measuring the secondary current I.sub.2. The secondary current I.sub.2 may be measured using a burden resistor that is placed across the secondary winding. In particular, the flow of the secondary current I.sub.2 through the burden resistor creates a voltage V.sub.2 across the burden resistor which can be used to determine the current I.sub.1 using the following relationship: ##EQU1##
where R.sub.b is the resistance of the burden resistor. As a result, the primary current I.sub.1 may be determined by measuring the voltage V.sub.2.
This arrangement is only usable, however, when the current transformer is not saturated, that is, when the amount of magnetic flux flowing the transformer core has not reached a maximum level. When the current transformer saturates, Eqs. (1) and (2) no longer apply.
The conventional solution to this problem is to use a current transformer with a higher volt-second rating. The volt-second rating is a measure of the amount of magnetic flux that can flow through the transformer core without the transformer core saturating. (Conventionally, the unit of magnetic flux is the Weber. However, for time-varying magnetic fields, the Weber is equivalent to the product of volts and seconds.) In practice, the volt-second rating is the main parameter which limits the dynamic range of a current transformer in terms of the magnitude and frequency of currents that may be measured. In general, the lower the frequency of the primary current, and/or the higher the magnitude of the primary current, the more likely the current transformer is to saturate.
Current transformers may be constructed so as to have a higher volt-second rating by increasing the cross sectional area of the transformer core. However, increasing the cross sectional area of the transformer core increases the size and cost of the current transformer. Moreover, as the frequency of the current which must be measured decreases and as the magnitude of the current which must be measured increases, a point is always reached at which it becomes impractical to construct the required current transformer. For example, to measure a DC current in steady state according to conventional approaches, a current transformer having a core with an infinite cross section would be needed. Of course, no such current transformer exists.
In general, most circuit designers use current transformers with the knowledge that Eqs. (1) and (2) apply so long as saturation is avoided. If saturation is a concern, then a current transformer with a larger core is utilized. However, little if any thought is ever given by most circuit designers to the fundamental problem of saturation.
In order to fully understand the problem of saturation, it is helpful to undertake a more detailed consideration of the operation of any current transformer. When the primary current I.sub.1 flows through the primary winding, the primary current I.sub.1 induces a magnetic flux .PHI..sub.1 that flows through the transformer core and that magnetically couples the primary winding and the secondary winding. The magnetic flux .PHI..sub.1 induces the secondary current I.sub.2 in the secondary winding, and the secondary current I.sub.2 itself also induces a magnetic flux .PHI..sub.2. Like the magnetic flux .PHI..sub.1, the magnetic flux .PHI..sub.2 flows through the transformer core and magnetically couples the primary winding and the secondary winding. If Eq. (1) was exactly true, the magnetic flux .PHI..sub.1 and the magnetic flux .PHI..sub.2 would cancel and no net magnetic flux would flow through the transformer core. In practice, however, slightly less current flows in the secondary winding than the amount predicted by Eq. (1). This difference is attributable, in general, to the fact that a non-zero voltage is developed across the secondary winding (due to the internal resistance of the secondary winding and the resistance of the burden resistor). As a result of the slight difference, there is a net magnetic flux .PHI. which flows through the transformer core. When the primary current I.sub.1 is a low frequency and/or high magnitude current, the magnetic flux .PHI..sub.1, and in particular the net magnetic flux .PHI., tends to drive the transformer core into saturation.
In some respects, the non-zero voltage that is developed across the secondary winding is a nuisance which causes measurement inaccuracies and which should be minimized to the extent possible. Perhaps ironically, however, it would be undesirable to eliminate the non-zero voltage altogether because the non-zero voltage is what permits the primary current I.sub.1 to be measured. Without any voltage developed across the secondary winding, no current would flow through the secondary winding and no voltage would be developed across the burden resistor. Consequently, it would not be possible to measure the primary current I.sub.1.
The voltage V.sub.2 that is developed across the secondary winding is related to the net magnetic flux .PHI. in the following manner: ##EQU2##
where N is the turns ratio (N.sub.1 /N.sub.2). (Although Eq. (3) is sometimes written with a minus sign before the right-hand term, a minus sign is not used herein.)
From Eq. (3), it is seen that the voltage V.sub.2 that is developed across the secondary winding is proportionally related to the change in the net magnetic flux .PHI. with respect to time. In saturation, since the net magnetic flux .PHI. stays at a constant saturated level, there are no time varying changes in the net magnetic flux .PHI.. As a result, there is no voltage developed across the secondary winding (V.sub.2 =0) and there is no secondary current that flows through the secondary winding (I.sub.2 =0). This is true even though current continues to flow in the primary winding (I.sub.1.noteq.0). It is therefore apparent that, in saturation, Eqs. (1) and (2) do not apply and the primary current I.sub.1 cannot be measured.
Other approaches do exist for measuring current. For example, another device which has been used to measure current is the Hall-effect sensor. In a Hall-type current sensing arrangement, the measured current induces a magnetic flux in a core in a manner similar to a current transformer. However, rather than using a secondary winding to sense the measured current, a Hall-effect sensor is placed in the path of the magnetic flux. The measured current is then measured by using the Hall-effect sensor to measure the magnetic flux in the core. However, since this arrangement also uses a core, it too is subject to the above-mentioned saturation problems.